Modern communications systems are ever more increasing in bandwidth, causing greater needs for broadband antennas. Some may require a decade of bandwidth, e.g. 100-1000 MHz. Some needs (e.g. military needs) may require broadband antennas for low probability of intercept (LPI) transmissions or communications jamming. Jamming systems can use high power levels and the antenna must provide a low voltage standing wave ratio (VSWR) at all times. The bandwidth need may be instantaneous, such that tuning may not suffice.
In the current physics, antenna size and instantaneous gain bandwidth may be limited through a relationship known as Chu's Limit (L. J. Chu, “Physical Limitations of Omni-Directional Antennas”, Journal of Applied Physics, Vol. 19, pp 1163-1175 Dec. 1948). Under Chu's Limit, the maximum 3 dB gain fractional bandwidth in single tuned antennas cannot exceed 200 (r/λ)3, where r is the radius of a spherical envelope placed over the antenna for analysis, and λ is the wavelength. While antenna instantaneous gain bandwidth is fundamentally limited, voltage standing wave ratio (VSWR) bandwidth is not. Thus, in some systems it may be necessary to trade away gain for increased VSWR bandwidth by introducing losses or resistive loading. Losses are required when the antenna must operate beyond Chu's relation, that is, to provide low VSWR at small and inadequate sizes. Without dissipative losses, the single tuned 2 to 1 VSWR bandwidth of an antenna cannot exceed 70.7 (r/λ)3.
Multiple tuning has been proposed as an approach for extending instantaneous gain bandwidth, e.g. with a network external to the antenna, such as an impedance compensation circuit. Multiple tuned antennas have complex polynomial responses, rippled like a Chebyshev filter. Although beneficial, multiple tuning cannot be a remedy to all antenna size-bandwidth needs. A simple antenna may provide a “single tuned” frequency response that is quadratic in nature, and Wheeler has suggested a 3π bandwidth enhancement limit for infinite order multiple tuning, relative single tuning (“The Wideband Matching Area For A Small Antenna”, Harold A. Wheeler, IEEE Transactions on Antennas and Propagation, Vol. AP-31, No. 2, Mar. 1983).
The ½ wave thin wire dipole is an example of a simple antenna. It can have a 3 dB gain bandwidth of only 13.5 percent and a 2.0 to 1 VSWR bandwidth of only 4.5 percent. This is near 5 percent of Chu's single tuned gain bandwidth limit and it is often not adequate. Broadband dipoles are an alternative to the wire dipole. These preferably utilize cone radiating elements, rather than thin wires, for radial rather than linear current flow. They are well suited for wave expansion over a broad frequency range, being a self exciting horn. A biconical dipole, having for example, a conical flare angle of π/2 radians has essentially a high pass filter response from a lower cut off frequency. Such an antenna provides wide bandwidth, and a response of 10 or more octaves is achieved. Yet, even the biconical dipole is not without limitation: the VSWR rises rapidly below the lower cutoff frequency. Low pass response antennas are seemingly unknown in the present art.
Broadband conical dipoles can include dissimilar half elements, such as the combination of a disc and a cone. A “discone” antenna is disclosed in U.S. Pat. No. 2,368,663 to Kandoian. The discone antenna includes a conical antenna element and a disc antenna element positioned adjacent the apex of the cone. The transmission feed extends through the interior of the cone and is connected to the disc and cone adjacent the apex thereof. A modern discone for military purposes is the model RF-291-AT001 Omnidirectional Tactical Discone Antenna, by Harris Corporation of Melbourne, Fla. It is designed for operation from 100 to 512 MHz and usable beyond 1000 MHz. It has wire cage elements for lightweight and ease of deployment.
U.S. Pat. No. 7,170,462, to Parsche, describes a system of broadband conical dipole configuration for multiple tuning and enhanced pattern bandwidth. Discone antennas and conical monopoles may be related to each other by inversion, e.g. one is simply the other upside down. U.S. Pat. Nos. 4,851,859 and 7,286,095 disclose such antennas formed with connectors at the cone and disc, respectively.
Folding in dipole antennas may be attributed to Carter, in U.S. Pat. No. 2,283,914. The thin wire dipole antenna included a second wire dipole member connected in parallel to form a “fold”. In FIG. 5 of U.S. Pat. No. 2,283,914 the folded dipole member includes a resistor for the enhancement of VSWR bandwidth. Without the resistor, bandwidth was not enhanced (relative an unfolded antenna of the same total envelope) but there were advantages of impedance transformation and otherwise. Resistor “terminated” folded dipoles were employed in World War II. Later, in U.S. Pat. No. 4,423,423 to Bush, a resistive load was described in a folded dipole fold member. Resistively terminated folded wire dipole antennas may lack sufficient gain away from their narrow resonances.
Conventional discone antennas have broad instantaneous bandwidth but rapidly rising VSWR at frequencies below cutoff. To obtain sufficiently low VSWR at low frequencies, they may be too physically large. The large size may cause insufficient pattern beamwidth at the higher frequencies, and there the pattern may droop or fall below the target. Accordingly, there is a need for a broadband antenna that provides a low VSWR at all radio frequencies, at small size, and that does not suffer from these limitations.